(1) Field of the Invention
The present invention relates to a dome for a rotorcraft lift rotor, and also to a rotorcraft provided with such a dome.
(2) Description of Related Art
A rotorcraft conventionally includes an airframe extending from a nose to a tail. The tail may comprise a tail boom carrying a fin and stabilizers.
The airframe may carry at least one rotor providing the rotorcraft with at least some of its lift and possibly with propulsion. Such a rotor is referred to below as a “lift rotor”, and sometimes as a “main rotor” by the person skilled in the art.
The tail also includes a tail fin, sometimes carrying a rotor for controlling yaw movement of the rotorcraft. As a result, this rotor is sometimes referred to as a “tail rotor”, given its location within the rotorcraft.
The airframe also has covers arranged under the main rotor. These covers may be movable covers serving to give access to a power plant, for example. Such covers are conventionally referred to as “engine covers”.
While the rotorcraft is flying in translation, air flows along the rotorcraft. The slipstream of air downstream from the lift rotor and the covers of the airframe is generally disturbed. Such disturbances can then impact against a fin and/or a stabilizer at the tail of the rotorcraft.
This disturbed slipstream is commonly referred to as the “wake”. This term is used below for reasons of convenience, even though it is not entirely appropriate. Specifically, in theory, a wake corresponds to a region downstream from an obstacle in a flowing fluid, with the fluid being at rest in that region relative to the obstacle.
The impact of the disturbances generated by a lift rotor on the tail of a rotorcraft can lead to one or more modes of vibration of the tail being excited aerodynamically, where such excitation is commonly referred to in aviation as “tail-shake”. Such excitation presents numerous drawbacks, and in particular:
in terms of comfort for the crew and for passengers;
in terms of fatigue for parts and equipment; and
in terms of operation of certain systems of the rotorcraft.
Furthermore, the air flow can become separated locally from the airframe downstream from a lift rotor, and in particular downstream from the engine covers. Such separation tends to amplify the magnitude of the excitation on the tail of the aircraft by increasing the amplitude and by enriching the frequency signature of such aerodynamic fluctuations.
In order to reduce such excitation, a dome may be arranged on the head of the lift rotor.
A dome is generally in the form substantially of a cap of an ellipsoid of revolution.
Furthermore, notches are arranged in a peripheral ring of the dome, in particular to avoid impeding the flapping and lead/lag movements of the blades.
Under such circumstances, a dome may comprise an ellipsoidal cap provided with one notch per blade of the rotor.
While flying in translation, this dome deflects the flow of air downstream from the lift rotor in a downward direction. The flow of air is then deflected mainly towards the covers and the tail boom, and no longer towards the stabilizer and the fin of the rotorcraft. The tail shake effect is thus decreased.
Furthermore, a dome tends to deflect the slipstream downwards downstream from a lift rotor.
In addition, the dome tends to limit the extent to which a downstream slipstream becomes separated from engine covers.
Nevertheless, such domes do not always perform well.
A dome is generally dimensioned so as to deflect a slipstream downwards, and to minimize said separation of the slipstream downstream from the lift rotor carrying the dome. This dimensioning tends to determine the diameter of the dome.
Under such circumstances, the dome does not provide any means for acting on the frequency signature of the slipstream that is generated, nor does it provide means for acting on the forces to which the dome is subjected. A manufacturer therefore cannot act on a dome of given shape in order to solve problems of interactions between the slipstream and the dome.
Documents U.S. Pat. Nos. 4,212,588, 4,281,967, EP 2 727 832, FR 2 863 583, and WO 2007 055813 are also known.